A tapered optical waveguide, in which an end portion of a waveguide layer formed on a substrate is tapered, is known (e.g. JP-A-60-78406). In such a tapered optical waveguide, guided light beam propagating in a waveguide layer having a uniform layer thickness is taken out towards the substrate at the tapered portion and it is known that the light beam thus taken out is divergent.
Therefore, if it were possible to change the diverging light beam taken out from the tapered optical waveguide towards the substrate into converging one so as to form an extremely small focus, it would be expected to use it e.g. as an illuminating spot for recording/reproducing information on and from an optical disc.
A structure utilizing a waveguide in an optical head used for recording/reproducing optical disc signals is known, in which a diffraction grating for optical coupling is formed on a plate type optical waveguide (JP-A-61-236037). Since in this type of optical heads a diffraction grating is used for taking out light to the exterior thereof or introducing reflected light in the optical waveguide, it has problems that e.g. light utilization effiency for the illumination on the optical disc is low, because the guided light is diffracted on both the sides of the diffraction grating, that it is difficult to fabricate the diffractioin grating for the coupling, etc.
On the contrary, the tapered optical waveguide has a high applicability to the optical head described above, because the light utilization efficiency thereof is high and its fabrication is easy owing to the fact that emission characteristics of the light taken out towards the substrate side are stable with respect to variations in wavelength of the light source and further substantially all of the emitted light can be taken out towards the substrate side, as described later.
However, even if it is possible to converge the emitted light beam so as to obtain an extremely small focus by designing suitably the tapered shape as described previously, if the focus is located in the closest neighborhood of the interface between the substrate and the waveguide layer, the light beam is divergent in the state where it is really taken out outside of the substrate and it is not possible to utilize directly the focus of the light beam thus taken out for the purpose of the illumination for the recording/reproduction of information from and on the optical disc.
Furthermore it may be required to form a substrate not only for the tapered optical waveguide but also for light integrated circuits, etc. consisting of regions having different refractive indices, as described later. In this case, if it is formed only by joining two substrates having different refractive indices by means of optical resin, a problem is produced that scattering, etc. of the guided light is generated at the joint portion.